Bimodal molecular weight distribution of a resin indicates that a resin comprises to components of different molecular weight, and implicitly requires a relatively higher molecular weight component (HMW) and low molecular weight (LMW) component. Physical blending of two different polymers will produce a bimodal blend. Such a concept appears in U.S. Pat. No. 4,461,873. Those physically produced blends contain high gel levels. Accordingly, their actual application in film production is of theoretical interest only because of film appearance due to those gels.
To replace physical blending, the art shows a development by which such bimodal resin blends are produced by process techniques, sometimes referenced as tandem process polymerization. Such a process relies on a two (or more) reactor set up; in one reactor, one of the two components of the bimodal blend is produced under a set of conditions maintained in the first reactor, and transferred to a second reactor, where under a different set of conditions from those in the first reactor the second component is produced, with a different molecular weight. Compared to bimodal HMW-high density polyethylene [HMW HDPE] produced by physical blending, blends produced in a tandem operation may have relatively improved film appearance. However, the film appearance is dependent on the HMW component being a major component, greater than 50 percent by weight fraction, of the blend. U.S. Pat. No. 4,307,209 discusses some of the constraints on composition which the tandem mode of operation imposes on product because of the presence of gels.
The problems which inhere in tandem produced products, as discussed in U.S. Pat. No. 4,307,209, may be due to a function of particles of product which have varying LMW/HMW ratios. It is difficult to avoid having particles that have not incorporated LMW species, particularly, when the HMW component is produced in the first reactor. This may be due to deactivation of the HMW reactor particles or because of their bypassing the LMW reactor. Such particles with fairly high viscosity appear as gels and would be incorporated (through deformation) only if the bulk matrix viscosity was high enough. The latter requires that the weight fraction of the HMW component be fairly high (greater than 55 percent).
The desirability of bimodal products resides in product strength and processability, which are provided by the individual components of the blend. Processability relates to the capacity of the resin to be used in existing lines efficaciously.